Structure of the myosin head

The force of muscle contraction is known to be produced by the interaction of a myosin head with an actin filament [Hynes et al., 19871.Production of work would require that a force of several piconewtons be exerted on an actin filament over a distance of approximately 10 nm.Production of force through a distance would require large conformational changes in the proteins that are involved [reviewed by Cooke, 1986; Vibert and Cohen, 19881.It is generally thought that these conformational changes involve the rotation of all or a portion of the myosin head.If the myosin head were to rotate as a rigid body, these conformational changes would occur in the structure of the actin filament or in the structure of the acto-myosin interface.Alternatively, only a portion of the myosin may rotate while another portion remains rigidly attached to actin.In this case the conformational changes that produce force could involve the relative movement of domains within the myosin head or could consist of a gradual deformation of the myosin head extending over a large portion of the molecule.The structure of the myosin head has been determined by electron microscopy of myosin, of myosin heads bound to actin, and of crystals of myosin heads.These studies have generally attained a resolution of 2-3 nm.All of these structural studies agree that the myosin is a pear-shaped molecule with a massive region at its distal end tapering toward the head-rod junction.In electron micrographs of crystals the mass at the distal end is approximately 6 nm in length and 4 nm in width, and it is separated from the region more proximal to the headrod junction (known as the neck region) by an indistinct cleft [Winkelmann et al., 19851.Recent electron micrographs of negatively stained or rotatory shadowed myosin molecules have revealed greater detail in the head portion of the molecule.Micrographs of negatively stained myosin heads showed two stain-filled clefts that run roughly perpendicular to